Alkynyl zincates

Alkynyl zincates (59), derived from 5-hexynyl tosylates, undergo a re-type endo cyclization to fonn cyclohexynes (60).58 This reaction takes place in competition with exo cyclization, leading to the formation of l-(cyclopentyhdene)alkylzincs (61). 

In this step, a new type of enantioselective alkynylation of a prochiral ketone promoted by a zinc alkoxide is carried out. By this, the alkyne moiety of zincate 11 undergoes re-attack (see Chapter 3) of the ketone 6 furnishing alcohol 12 with the i -configuration in 99.2 % ee and pyrrolidinyl norephedrine 10 after work-up. 

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